1. Field of the Invention
The present invention relates generally to connecting electronic devices in dc series, and in either rf series or parallel. More particularly, the present invention pertains to connecting electronic devices in dc series, equally, proportionally, and/or variably dividing a dc source voltage between, or among, the electronic devices, and rf decoupling the series-connected electronic devices.
2. Description of the Related Art
Frequently, maximum operating voltages of solid-state electronic devices are too low for the dc source voltage that is available. By connecting the solid-state electronic devices in dc series, the dc source voltage may be divided between, or among, a plurality of solid-state electronic devices, either equally or proportionally, as desired, thereby providing dc voltages that are usable for any desired type of solid-state electronic device. Because, in schematic drawings, the solid-state electronic devices are shown stacked on top of each other, this type of circuit has come to be known as a “totem pole” circuit. Herein this type of circuit is called a “shared-current electronic system.”
Further, the dc source voltage may be divided variably, as will be shown subsequently. In addition, as will be shown subsequently, other types of electronic devices, such as oscillators, may be connected in a totem-pole arrangement, either with or without solid-state electronic devices. The inputs and outputs of the electronic devices, whether rf or lower frequencies, may be connected in either series or parallel.
As an example of equal or proportional division of the dc source voltage, gallium arsenide field-effect transistors (GaAsFETs) are the primary solid state devices used for amplification of high frequency signals in the range of 3 GHz and higher. GaAsFETs have the advantages of being readily available and relatively inexpensive. However, a major disadvantage of GaAsFETs is that the maximum operating voltage is commonly +10.0 volts dc.
For many transmitter/amplifier applications, particularly airborne applications, the dc supply voltage is 28.0 volts dc, plus or minus 4.0 volts dc. Since gallium arsenide FETs have an operative voltage of +10.0 volts dc, the use of gallium arsenide FETs has presented a problem.
Traditionally, there have been two solutions to this problem. One is to use a linear voltage regulator. The other is to use a switching regulator.
In linear voltage regulators, the voltage is linearly regulated from the supply of 28.0 volts to approximately 10.0 volts with the power difference being dissipated in heat by the regulator. This type of regulation has the disadvantages of excessive heat generation and low power efficiency.
Switching regulators, on the other hand, are power converters that transfer the power of a higher voltage supply to a lower voltage with increased current capacity. This type of regulation has the advantage of low heat dissipation and high power efficiency, but has the disadvantages of increased cost; space inefficiency, due to large size; and the creation of a spurious signal on the rf carrier (EMI problems) due to the switching action of the regulator. A high-attenuation filter is required to suppress this spurious switching signal.
A third approach to solving the problem of disparity between the operating voltage of solid-state devices and a dc source voltage has been to connect the solid-state electronic devices in dc series, thereby dividingly sharing the dc source voltage and utilizing the same current flow. This shared-current approach was presented in IEEE Transactions on Microwave Theory and Techniques, Volume 46, Number 12, of December 1998, in an article entitled “A 44-GHz High IP3 InP-HBT Amplifier with Practical Current Reuse Biasing.”
Shared-current electronic systems solve the problem of the disparity between the operating voltage of solid-state devices and a higher supply voltage. Two or more solid-state electronic devices are connected in series for dc operation, but they are connected in parallel for rf operation.
That is, current that flows in series through the solid-state devices is used two, or more, times in the production of the rf output. The dc current is used once in each of two, or more, series-connected solid-state electronic devices, thereby increasing the rf output for a given current flow, as compared to rf amplifiers connected in the conventional fashion.
However, shared-current electronic systems have been used only at low rf powers, as in the above-referenced article wherein the power was in the order of 100.0 milliwatts. At higher rf powers, problems associated with inadequate rf decoupling have included low power efficiency, oscillation, a decrease in reliability of the circuits, and destruction of the solid-state devices.
The problem of rf decoupling intensifies for rf amplifiers that operate over a wide band of frequencies. However, in U.S. patent application Ser. No. 10/028,844, which is incorporated herein by reference thereto, Lautzenhiser et al. solve the problem of rf decoupling for both narrow-band and wide-band rf amplifiers.
While one reason for connecting solid-state devices includes the low operating voltages of GaAsFETs, some dc source voltages are too high for other types of solid-state devices. For instance, some telephone systems operate at 50.0 volts, which is too high for many solid-state electronic devices, such as some bipolar-junction transistors, some MOSFETs, and some J-FETs. Therefore, a basic reason for connecting solid-state devices in dc series is to provide a compatible operating voltage without incurring the power loss of a linear regulator or the noise of a switching regulator.
Another reason for connecting solid-state devices in series is to variably proportion the dc source voltage between, or among, two or more solid-state electronic devices.
The dc source voltage may be variably proportioned for the purpose of variably shifting or proportioning, or even rapidly switching, rf power from one antenna to another, as taught by Lautzenhiser et al. in U.S. patent application Ser. No. 10/177,572, filed Jun. 21, 2002, which is incorporated herein by reference thereto.
The dc source voltage may also be variably proportioned between, or among, two or more solid-state electronic devices for the purpose of phase shifting an rf output as taught by Lautzenhiser et al. in U.S. patent application Ser. No. 10/091,056, filed Mar. 4, 2002, which is incorporated herein by reference thereto.
Further, as taught herein, a solid-state electronic device, such as a FET, may be connected in dc series with a processing electronic device, such as an oscillator or a baseband processor, that may include hundreds of discrete components. By dc series-connecting the solid-state electronic device and the processing electronic device in dc series, and proportionally dividing the dc source voltage between the devices, a dc voltage suitable for each device is provided, the dc current is shared, the use of a voltage regulator is obviated, and power efficiency is increased greatly.
An important use of shared-current electronic systems is in spectrally efficient digital modulation systems such as SOQPSK (Tier I) or multi-h CPM (Tier II) in which the quantity of data in a given bandwidth is doubled or tripled respectively as compared to the PCM/FM (Tier O) waveform. Importantly, the shared-current principle also increases the power efficiency of electronic systems that use Tier O and Tier II waveforms, since all three waveforms (Tier O, Tier I, and Tier II) may be produced by the same hardware by making a change in the software.
In shared-current electronic systems, problems with rf decoupling are most severe between the solid-state devices. For instance, when using FETs, decoupling is the most critical with regard to a source terminal of any FET that is connected to a drain terminal of a next-lower FET. Capacitors and rf chokes are used for rf decoupling and rf isolating, but selection and design of capacitor decoupling is the most critical.
The next most critical location for rf decoupling is the source terminal of the lower FET when the source terminal of the lower FET is connected to an electrical ground through a resistor, as shown herein. However, if a negative bias voltage is used for the gate of the lower FET, and the source is connected directly to an electrical ground, this source terminal is already rf decoupled. Again, selection and design of capacitors used for rf decoupling is critical.
Other critical rf decoupling problems are those associated with the supply voltage to the drain of the upper FET and bias voltages to the gates of the FETs. The use of properly designed rf chokes are sufficient to provide adequate rf decoupling in these locations.
Unless rf decoupling is provided as taught herein, reduced efficiency will certainly occur, and both instability and destruction of the solid-state electronic devices may occur. More particularly, if one of the solid-state electronic devices goes into unstable self-oscillation, it will consume more dc bias and most likely become over biased resulting in destruction of the solid-state device.
In a shared-current configuration that uses FETs, all FETs may be destroyed if one FET fails, depending on how the first FET fails. For example, if the upper FET oscillates and consumes the dc bias, it will be over-biased and will be destroyed. If, in the destruction, the drain and source short circuit, which is a common type of failure, the lower FET will be over-biased, too, so that the lower FET will also fail.
In short, inadequate rf decoupling, at the very least results in poor efficiency. At the worst, and with higher likelihood at higher rf outputs, it results in destruction of the FETs and/or damage or destruction of circuits connected to the FET inputs and outputs.